1. Field of the Invention
This invention relates generally to electronic equipment adapted for use in environments which are hazardous because of the presence of flammable gases, more particularly relates to an RF feedthrough connector or sealing fitting for use with explosion proof enclosures, and most particularly is directed to a compact antenna and feedthrough assembly which is leakproof and therefore can be mounted near the hazard to facilitate wireless data communication with the hazardous site.
2. Description of the Related Art
Areas near or surrounding facilities for the manufacture, storage, transportation, or use of flammable materials are hazardous environments because of the possibility of accidental leakage and ignition by a spark or flame in the environment. Consequently, there are regulations and standards of construction for buildings and equipment in order to minimize the associated risks and make them explosion proof. Explosion proof means that they will not cause an explosion and, if one occurs within an explosion proof enclosure, it will be safely contained and the pressure safely relieved. The regulations include sealing requirements so that hazardous gases cannot reach an electric arc or spark and thereby ignite a fire.
The most common method to isolate a potential ignition source in a hazardous location is through the use of an explosion proof enclosure rated as NEMA 7, 8, 9, or 10 which is strong enough to withstand the pressure of an explosion in the interior of the enclosure. Such an enclosure must be designed to safely vent the hot gases resulting from an explosion in a way that the gases are cooled below the ignition temperature of an explosive mixture outside the enclosure. In order to contain the pressures of an internal explosion, the enclosure is typically constructed of heavy cast steel or aluminum. Areas around various hazardous installations are ranked, based on distance, such as Class 1/Division 1 and the closer equipment is located to the hazard, the more rigorous are the regulations. There are regulations for electronic equipment, which must meet the rating standards for the place where it is located.
Often it is desirable to have wireless monitoring and control systems associated with equipment in hazardous environments such as oil rigs, fuel or other flammable-containing tanks or power plants. A wireless system avoids the need for installing long wires or transmission lines. However, a wireless system requires at least one antenna which is used for wireless data transmission to and from the monitoring and control equipment. Unfortunately, a wireless antenna can not be located within the explosion proof enclosure because the metal enclosure would shield the antenna and prevent transmission of the electromagnetic waves to a remote antenna in communication with the monitoring and control system.
It would be desirable to attach the antenna to the exterior of the explosion proof enclosure to permit radio communication. However, the use of RF transmitters in a hazardous or potentially explosive environment could cause a spark and/or a flame path out of instrument housing containing a transmitter or transceiver and associated instruments. Currently, no antenna is rated for Class 1, division 1, which is a location close to the hazard. The result is that, in the prior art, coaxial cable must extend to a location far enough away from the hazard that an available antenna that will meet the standards for the greater distance can be used.
The use of wireless monitoring and control systems in hazardous environments has always presented a problem. The exposed antenna creates a series of problems since, in the prior art, the typical antenna is attached to a coaxial connector outside of the explosion proof enclosure and is displaced a substantial and sufficient distance from the hazard and from the explosion proof enclosure that it will meet the less restrictive regulations and standards for the greater distance. Some manufacturers of wireless data systems have utilized a conduit with a sealed conduit fitting and having a coaxial connector at one end for the antenna connection and a ¾″ NPT thread at the other end. The sealed fitting utilizes a combination of both fibrous packing and sealing compound to seal the connector and coaxial cable allowing the fitting to be attached to the enclosure and allowing for a length of cable and a coaxial connector to be connected at one end to the transmitter inside the enclosure and at the other end to the antenna displaced from the enclosure.
It is an object and feature of the present invention to provide an explosion proof structure which permits an antenna for a wireless system to be attached directly to the explosion proof enclosure or pipe so that it can be located nearer to the hazard, avoid the need for long lengths of transmission line extending a greater distance from the hazard and yet could be certified by one or more of the licensing agencies for use close to the hazard.
It is a further object and feature of the invention to provide an antenna having an antenna/cable interface entering the explosion proof enclosure that has a sealing process and structure that does not allow the hot gases to be able to escape from the explosion proof enclosure through this path so that it maintains the explosion proof integrity of the enclosure.
It is a further object and feature of the invention to provide a coaxial cable connector for feeding through the wall of the explosion proof enclosure that has the above properties and can be used to connect to a coaxial cable on opposite sides of the wall of the explosion proof enclosure, instead of being connected on one side to an antenna, so that it may be used for a transmission line feedthrough or bulkhead connector were desired.
It is a further object and feature of the invention to provide a coaxial connector interface that may be connected to a pipe or housing containing the coaxial cable carrying the RF signal to an antenna. The interface is sealed in such a manner as to withstand the pressure of an explosion inside an instrument housing containing a RF transmitter or transceiver preventing hot gases from escaping through the antenna connection. The RF radiating element is DC grounded to prevent a potential spark that could take place at the radiating element.